1. Technical Field
This disclosure relates to a beamformer for providing a weight vector to a received signal in mobile communication environments, e.g., Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), Orthogonal Frequency Division Multiple Access (OFDMA) and WiMAX or WiBro mobile communications, etc. More specifically, this disclosure relates to a weight vector computing part having such a beamformer that is applicable to a smart antenna system, an apparatus and method for demodulating the received signal, and a computer-readable recording medium for recording a program that embodies the method.
2. Related Art
The importance of smart antenna system is widely recognized as a tool for efficiently increasing the capacity of mobile communication system. In general, it has been known that the smart antenna system enhances the performance of wireless communication system through a spatially selective reception, which causes the interfering signals to be reduced or solved.
In conventional cases, the weighting information needed in smart antenna system is obtained from a traffic channel in reverse link. The intrinsic characteristic of the traffic channel that the integration interval for despreading procedure cannot be taken long enough, however, the information cannot help being deteriorated.
In order to solve this problem, there was a Korean Patent No. 239177 registered on Oct. 19, 1999 entitled, “Receiving Apparatus and Method of Smart Antenna using Pilot Signals in CDMA Mobile Telecommunication System”. In the Korean patent 239177, the signal from each antenna element is multiplied by an adaptive weight and combined to produce array output. The output is despreaded with the PN code and properly filtered. In order to produce a reference signal, however, the filtered signal is multiplied again by the same PN code. The technique disclosed in the patent 239177 is based on an error signal produced from the difference between the array output and the reference signal. The adaptive procedure produces the weights to be multiplied by the received signals from a plurality of antenna elements by minimizing the error signal by means of well-known technique, LMS (least mean square) algorithm.
The problem in this method, however, is in executing the LMS algorithm which essentially needs the reference signal for producing the difference to be minimized at each adaptation step. In fact, the procedure of multiplying the PN code to the despreaded received signal is needed only to produce the reference signal. It is absolutely irrelevant to the reception or demodulation procedure itself. The procedure of producing the reference signal must cause additional delays and errors as well as the complexity due to PN code multiplication.
As another technique, there is a Korean patent application 1999-28020 which has been published on Feb. 5, 2001, entitled “CDMA Signal Demodulator for an adaptive smart antenna system”. In this disclosure, CDMA signal demodulator in the adaptive smart antenna system consists of a searcher bank for PN code acquisition, a finger bank which produces accurate timing information, a correlator bank which produces the despreaded data, a beamforming parameter computer which produces the weights to be multiplied to received signal, a beam-forming multiplier which produces the array output by multiplying the weights to the received signal, and a multipath combiner which aligns the despreaded data in time domain.
The problem in this technique is that it does not provide how each block (or, equivalently, bank) co-operates and interfaces to each other. One practical phenomenon caused by that defect is the initial PN code acquisition cannot be achieved. In order for the smart antenna system shown in the Korean patent application 1999-28020 to work as properly as claimed, it should be assumed that all the timing information needed at each block must be provided externally, which is not true in real situations. For example, both initial PN code acquisition and multipath searching should somehow be achieved apriori at the beginning stage, which is never realistic in practical situations, for the correlator bank to work properly. More specifically, the technique disclosed in the Korean patent application 1999-28020 fails to provide how the timing information is provided from the searcher bank to the correlator bank and the beamforming parameter computer such that the correlation for despreading the received signal and computing the weights at a proper time cannot be achieved. Also, without specifying the detailed method of PN code acquisition, multipath combining, which is essential in CDMA receiver, can never be achieved as well.
Therefore, the system shown in the Korean patent application 1999-28020 must start with inaccurate timing information at the beginning stage, which results in extremely adverse reliability. In addition, inaccurate timing information results in tremendously slow convergence in adapting the weights even if it can converge. Mostly, it does not converge at all.
Another technique is disclosed in a Korean patent application 1999-30463 published on Feb. 15, 2001, entitled “Smart Antenna System having a beam-former and an adaptive equalization combiner for multipath signals”. The main part of this technique is that a beamforming can be achieved through an adaptive beamforming algorithm instead of selection diversity through the searcher bank.
In this technique as well as in the previous ones, there is no explanation about how the timing information is obtained. More specifically, it is assumed in this technique that the searching is perfectly obtained apriori at the beginning stage, i.e., stage before the despreading procedure for the chip-level weighting, which is never true in real situations. As in the previous case, i.e., 1999-28020, the inaccurate timing information due to the lack of specified searching technique must cause very slow converge in the adaptation of weights even if the procedure does not diverge. The slow convergence leads to a serious degradation in performance of smart antenna system.
In addition, this technique fails to disclose for any normal people with common knowledge to understand how the adaptive beamforming algorithm searches for the weights.